The traditional view has always been a combination of genetics and behavior factors, namely reading and other close up work during the age of 8-14 when the eye is still capable of axial elongation. The light intensity theory is a more recent development which has some merits, but more evidence is needed for proof.

After WW2, Canadian Inuits started to send their children to school for the first time in history and myopia became endemic from that generation onwards.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1956268/

And to put the source in context:

- This was before the age of computers and smartphones so screen time was much less of a factor.

- Inuits traditionally spend a disproportionate time indoors due to the climate.

- The schooling was likely shorter and less stressful than those found in places like Japan and Korea, yet the effects are just as apparent.

Similar rises in myopia rate was observed when daily schooling was introduced to many other places, but few occur on the same scale. Thus it is possible that Asians and Inuits are genetically predisposed to myopia and it took until the modern age for this to become a problem.